Recently, interests in energy storage technologies have increased. As the energy storage technologies are extended to such devices as cellular phones, camcorders and notebook PCs, and further to electric vehicles, the demand for a high energy density battery used as a power source of such an electronic device has increased. A lithium ion secondary battery is one of the most satisfactory batteries, and numerous studies towards improvements are now in progress actively.
A lithium secondary battery may be manufactured with various shapes. Representative examples include an angled lithium secondary battery, a cylindrical lithium secondary battery and a pouch-type lithium secondary battery.
As shown in FIG. 1, a pouch-type lithium secondary battery 10 generally includes an electrode assembly 11, and a case 12 defining a space for receiving the electrode assembly 11. The electrode assembly 11 includes an anode made of carbon material capable of occluding or emitting lithium ions, a cathode made of lithium-containing oxide, and a separator interposed between the cathode and the anode to electrically insulate them. Cathode and anode taps are respectively drawn from one edges of the cathode and the anode of the electrode assembly 11 to configure tap units 13, which are electrically connected to each other with being aggregated on each electrode plate. The tap units 13 are respectively welded with electrode terminals, which may be connected to an external terminal.
The pouch-type lithium secondary battery 10 includes the pouch-type case 12 made of a sheet, which sheet is formed by laminating a polymer film on a metal sheet made of aluminum, for example. The case 12 has a space in which the electrode assembly 11 may be placed, and conventionally upper and lower cases are releasably joined to each other. In the pouch-type lithium secondary battery 10 configured as above, the electrode assembly 11 is installed in the pouch-type case 11 having a space, and then an electrolyte is injected therein. And then, heat and pressure are applied around the pouch-type case 12 to firmly seal the pouch-type case 12, thereby completing the pouch-type lithium secondary battery 10.
As mentioned above, the pouch-type lithium secondary battery employs a pouch-type case made of sheet, so it is possible to fabricate a light lithium secondary battery in various shapes through a simple manufacturing process. However, since the pouch-type case is used, the lithium secondary battery therein may be more vulnerable to swelling due to increased inner pressure as compared to a cylindrical or angled battery. Along with the swelling, the thickness of the battery is increased, causing problems in electronics such as a cellular phone or a notebook and also creating serious influences on the stability and performance of the battery.
This swelling of the battery is more serious in a lithium secondary battery to which a non-aqueous electrolyte containing a dimethyl carbonate is injected.
For example, Japanese Patent No. 3,032,338 discloses a non-aqueous electrolyte secondary battery containing a ternary system organic solvent composed of ethylene carbonate, dimethyl carbonate and methyl propionate. The non-aqueous electrolyte having dimethyl carbonate as a linear carbonate exhibits good high-rate discharging characteristics. However, since dimethyl carbonate has a low boiling point, the battery may significantly swell if the battery is overheated or left alone at a high temperature. If ethyl methyl carbonate or dimethyl carbonate is added as linear carbonate or if only a small amount of dimethyl carbonate is added, the swelling problem of the battery may improve, but high-rate discharging characteristics deteriorate.